In general, as illustrated in FIG. 1, in the case of an excavator as a construction machine, an upper turning body 13 of a main body 10 is rotatably installed by a turning motor in a state in which a turning bearing 12 is disposed between the upper turning body 13 and a lower traveling body 11 of the main body 10.
A power device 14, a cabin 15, and a front working device 16 are mounted at a front side of the upper turning body 13 of the main body 10, the front working device 16 is pivotally attached to the upper turning body 13 so that a boom 17 may be freely rotated in an up and down direction, an arm 18 is freely rotatably and pivotally connected to the boom 17, and a bucket 19 is freely rotatably and pivotally connected to the arm 18.
Further, the boom 17 is rotated in the up and down direction by a boom cylinder 17c, the arm 18 is rotated by an arm cylinder 18c, and the bucket 19 is rotated by a bucket cylinder 19c. A fluid for operating the respective cylinders is oil, that is, hydraulic oil.
Meanwhile, a regeneration control valve block 20, which is provided with a plurality of valves that constitute an energy regeneration system for regenerating boom energy released from the boom cylinder 17c when the front working device 16 is lowered, is attached to a lower rear surface of the boom 17.
In the case of the boom energy regeneration control system, when the boom 17, which has been raised, is lowered when the front working device 16 moves vertically, hydraulic oil at high pressure is discharged from a head of the boom cylinder 17c by potential energy of the boom 17.
Because the hydraulic oil becomes useless if the hydraulic oil discharged at high pressure returns to a tank as it is, the hydraulic oil discharged at high pressure from the head of the boom cylinder 17c is accumulated in regeneration devices such as an accumulator, or used to rotate a separate hydraulic motor to supplement output from an engine, thereby reducing fuel consumption of the engine. In a case in which the excavator performs any load work, the hydraulic oil accumulated in the accumulator is discharged, thereby effectively utilizing potential energy of the boom 17.
However, there is a problem in that in a case in which some of the regeneration devices, which constitute the boom energy regeneration control system, have an abnormality and thus cannot be normally operated, the boom cylinder cannot be normally operated when the boom is lowered, which causes inconvenience for an operator.